Fluorosilicone coatings

ABSTRACT

The present invention relates to the use of a thin fluorosilicone elastomer coating as a cost effective solution to the oil weepage problem while retaining the sealing advantages of silicone rubber.

This is divisional of application Ser. No. 08/273,515 filed on Jul. 11,1994 now U.S. Pat. No. 5,616,403.

The present invention relates to fluorosilicone coatings. Moreparticularly, the present invention relates to a method of coatingfluorosilicone elastomer on a nonfluorosilicone substrate tosubstantially reduce oil permeability.

BACKGROUND OF THE INVENTION

Over the past ten years, molded silicone rubber has been used in variousautomotive sealing applications such as valve rocker cover and oil pangaskets. The reason for this development is to be found in theoutstanding properties of silicones. As is well-known, silicones havehigh temperature stability and cold temperature flexibility inconjunction with low stress relaxation and ease of fabrication. Heatcured silicone rubber compounds have a proven track record in enginesealing applications and have provided the longevity of service requiredby the automotive community.

Recently the automotive manufacturers have experienced some problems inoil permeability, known as "weepage". Silicone rubber gaskets have apropensity to weep motor oil through the rubber, resulting inaccumulation of oil on the outside surface of the engine. Although thisdoes not result in gross leakage, it causes an unsightly slick which hasbecome a significant component of engine warranty costs.

One approach which has been shown to moderately reduce weepage in methylvinyl silicone compounds is the incorporation of relatively largeamounts of extending fillers. Unfortunately this adversely impacts thephysical properties and renders gaskets molded from these materialsunsuitable for use in cam cover sealing applications which use plasticcomposite covers. This is due to unacceptable increase in compressivemodulus which causes the covers to crack under load. In addition, thereduction in weepage is only partial which does not satisfactorilyresolve the problem.

Another approach is to use gaskets made from fluorosilicone elastomers.It has been shown by the major silicone suppliers and gasket fabricatorsthat fluorosilicone elastomers as a special category of silicones do notexhibit oil weepage. However, gaskets from fluorosilicone elastomers aremore expensive to fabricate than gaskets from silicone gaskets. Furthersince fluorosilicones are halogenated elastomers, disposal of thefluorosilicone gaskets may give rise to environmental concerns. Thusthere is a need for automobile gaskets which minimizes thefluorosilicone content but offers substantial reduction to oil weepage.

SUMMARY OF THE INVENTION

The present invention is based upon the discovery that a thinfluorosilicone elastomer coated on a substrate can substantially reducethe oil permeability of the substrate.

According to the present invention, a method for substantially reducingoil permeability, comprising the steps of

(a) applying a curable fluorosilicone coating to a substrate; and

(b) curing the fluorosilicone coating on the substrate.

The fluorosilicone coating comprises vinyl-containing fluorosilicone,hydrogen-containing siloxane, and a catalytic amount of platinum grouphydrosilation catalyst. In addition, an effective amount of organicperoxide can be added to improve the state of cure.

The present invention is further directed to a substrate withfluorosilicone elastomer coated thereon, wherein the fluorosiliconecoating has a thickness of 0.1-10 mil.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The fluorosilicone coating of the present invention comprisesvinyl-containing fluorosilicone composition, hydrogen-containingsiloxane and a catalytic amount of platinum group hydrosilationcatalyst. The vinyl-containing fluorosilicone composition can be eitherfluorosilicone homopolymers or fluorosilicone copolymers whosefluorosilicone contents are at least 80 mole percent in order to exhibita desired high resistance to oil permeation, more preferably more than95 mole percent, and most preferably 99 mole percent.

A hydrogen-containing siloxane is a component of the fluorosiliconecoating. The hydrogen-containing siloxane can be any of the currentlyknown organohydrogenpolysiloxanes having an average of at least two,preferably three silicon-bonded hydrogen atoms in each molecule andfunctions as a crosslinker for the fluorosilicone composition.

In order to form a curable rubber, additional cure catalysts areprovided. The catalysts include all of the well know platinum-grouphydrosilation catalysts which are substantially free of inhibitoryeffect and are effective for catalyzing the reaction of silicon-bondedhydrogen atoms with silicon-bonded alkenyl radicals or hydroxylradicals. A particular useful platinum catalyst is the platinum-siloxanecatalyst, substantially free of inhibitory impurities and detectableinorganic halogen, disclosed by Ashby et al. in U.S. Pat. No. 4,288,345(hereinafter "the Ashby platinum"), hereby incorporated by reference.

In addition, an organic peroxide free radical initiator or curing agentcan be provided to improve the state of cure. The preferred peroxidecuring agents are thermal decomposition organic peroxides convenientlyused to cure silicone elastomers. Examples of suitable organic peroxidefree radical initiators for use in the present invention are disclosed,for example, in U.S. Pat. No. 4,537,357 to Bobear which is incorporatedherein by references. Suitable peroxide catalysts include dialkylperoxide such as di-tertiary-butyl peroxide,tertiary-butyl-triethylmethyl peroxide,di-tertiary-butyl-tertiary-triphenyl peroxide, t-butyl perbenzoate and adi-tertiary alkyl peroxide such as dicumyl peroxide. A particular usefulperoxide catalyst is Diacyl peroxides such as benzoyl peroxide and 2,4dichloro benzoyl peroxide. The peroxide catalyst is used in a catalyticamount which is typically from about 0.8 to about 4.0, preferably fromabout 0.9 to about 3.0, and most preferably from about 2.0 to about 3.0,parts by weight per 100 parts of the fluorosilicone composition.

The fluorosilicone coating typically, but not necessarily, furthercomprises an appropriate quantity of a solvent or combination ofsolvents to provide a lower viscosity for, and hence an easierapplication of, the composition to various substrates. The amount ofsolvent is not critical; however, only a sufficient amount to facilitatethe preparation, handling and applying the composition is recommended.The solvent can be any liquid that will dissolve the fluorosiliconecomposition and other components without adversely reacting therewith.Preferred solvents include halogenated solvents, such aschlorofluorocarbons, esters, such as ethyl acetate, ketones such asmethyl ether ketone, ethers such as dibutyl ether. In addition, asolvent or combination of solvents can be used to dilute thefluorosilicone to render it sprayable.

The compositions of this invention may further comprise filler, e.g.,reinforcing filler, such as fumed silica or precipitated silica; and/orextending fillers, such as zinc oxide, titanium oxide, diatomaceousearth, crystalline silica and the like. A particularly useful filler forthe composition and method herein is a silica filler, preferably fumedsilica.

The fluorosilicone coating of this invention can be applied to asubstrate by any conventional means known in the art, e.g., by immersingthe substrate in the fluorosilicone coating, or by spraying thefluorosilicone coating to the substrate. The fluorosilicone coating hasa thickness of 0.1-10 mil, and more preferably 0.5-3.0 mil.

The fluorosilicone coatings of the present invention can be cured by anymeans which is known in the art. For example, curing of the coatings ofthis invention can be accomplished by heating the coated substrate attemperatures of 350° F.-450° F. for 5-10 minutes. Certain derivatives ofthe compositions may also be cured by ultraviolet light or electron beamradiation. For example, UV curable epoxy-functional fluorosilicones havebeen disclosed by Eckberg et al. in U.S. Pat. No. 5,178,959, herebyincorporated by reference.

In order that those skilled in the art might be better able to practicethe present invention, the following examples are given by way ofillustration and not by way of limitation. All viscosities were measuredat 25° C. Unless otherwise indicated, all parts are by weight.

EXAMPLE 1

Preparation of Fluorosilicone Solution A

10 grams of a 60 durometer fluorosilicone elastomer with approximatecomposition 65% fluorosilicone homopolymer, 4% methyl vinyl siliconepolymer, 23% fumed silica, 6% siloxane fluid, and 2% 2,4-dichlorobenzoyl peroxide paste (50% active) were dispersed in 90 grams of methylethyl ketone. The mixture formed a stable dispersion with solutionviscosity of 25 centistokes. A 1"×2"×0.075" sample of cured 45 durometermethyl vinyl silicone rubber was briefly immersed in this solution,removed and allowed to air dry for 5 minutes. The sample was then placedin an oven at 177° F. for 15 minutes, removed from the oven and allowedto cool to room temperature. The methyl vinyl silicone rubber was evenlyencapsulated in a fluorosilicone coating of thickness 0.5 mil-1.0 mil.The coating was partially cured with tacky surface and could be abradedwith moderate pressure.

EXAMPLE 2

Preparation of Fluorosilicone Solution B

10 grams of a 60 durometer fluorosilicone elastomer with approximatecomposition 64% fluorosilicone homopolymer, 4% methyl vinyl siliconepolymer, 22% fumed silica, 6% siloxane fluid, 4% methyl hydrogensiloxane fluid, and 0.3% Ashby platinum catalyst solution were dispersedin 90 grams of methyl ethyl ketone. The mixture formed a stabledispersion with solution viscosity of 21 centistokes. A 1"×2"×0.075"sample of cured 45 durometer methyl vinyl silicone rubber was brieflyimmersed in this solution, removed and allowed to air dry for 5 minutes.The fluorosilicone coating was uniform, continuous and of similarthickness to that stated in Example 1 above. The sample was then placedin an oven at 177° F. for 15 minutes, removed from the oven and allowedto cool to room temperature. The coating was observed to be tightlycured with very little surface tack, could not be abraded, and adheredtenaciously to the surface of the methyl vinyl silicone rubbersubstrate.

EXAMPLE 3

Preparation of Fluorosilicone Solution C

10 grams of a 60 durometer fluorosilicone elastomer with approximatecomposition 63% fluorosilicone homopolymer, 4% methyl vinyl siliconepolymer, 21% fumed silica, 6% siloxane fluid, 4% methyl hydrogensiloxane fluid, 2% 2,4-dichloro benzoyl peroxide paste and 0.3% Ashbyplatinum catalyst solution were dispersed in 90 grams of methyl ethylketone. The mixture formed a stable dispersion with solution viscosityof 21 centistokes. A 1"×2"×0.075" sample of cured 45 durometer methylvinyl silicone rubber was briefly immersed in this solution, removed andallowed to air dry for 5 minutes. The fluorosilicone coating wasuniform, continuous and of similar thickness to that stated in Example 1above. The sample was then placed in an oven at 177° F. for 15 minutes,removed from the oven and allowed to cool to room temperature. Thecoating was observed to be extremely tightly cured with no surface tack,shiny in appearance, could not be abraded, and adhered tenaciously tothe surface of the methyl vinyl silicone rubber substrate.

The results illustrate that the system is not curable by peroxide alone.Platinum catalyzed systems yield satisfactory tight cure. As shown inExample 3, peroxide and platinum addition cure together yields excellentresult.

EXAMPLE 4

    ______________________________________                                        Effect of Platinum-Group Catalyst Level on Degree of Cure                     ______________________________________                                                         Ex. 2  Comparative Ex. 2                                     ______________________________________                                        Fluorosilicone coating                                                                         B      B'                                                    Ashby platinum (grams)                                                                         0.007  0.021                                                 ______________________________________                                                         Ex. 3  Comparative Ex. 3                                     ______________________________________                                        Fluorosilicone coating                                                                         C      C'                                                    Ashby platinum (grams)                                                                         0.007  0.021                                                 ______________________________________                                    

The fluorosilicone coatings in comparative Examples 2 and 3 are moretightly cured than that in Examples 2-3. The fluorosilicone coating incomparative Example 3 is the best cured with no surface tack. Thiscoating could not be scratched off. The result clearly demonstrates thathigher platinum level is desirable for superior cure and the dualcatalyst system (the combination of platinum and the peroxide) optimizesfinal cure.

EXAMPLE 5

A modified ASTM D-814 Rubber Property-Vapor Transmission of VolatileLiquids test was used to assess the effectiveness of the coatingdescribed in Example 3 in reducing weepage through methyl vinyl siliconeelastomer. This test measures the permeability of liquids through anelastomeric membrane and reports the weight in grams of liquid dropletswhich form on the opposite surface of an elastomeric seal exposed to thetest fluid on one side. Test apparatus consists of a one half pintcanning jar equipped with a Kerr-type screw cap which encloses a disk ofthe sealing material die-cut from a flat sheet of the test elastomer.The jar was first filled with 100 ml of the test fluid, the seal affixedin place, and the test assembly then inverted such that the test fluidcontacts the inside surface of the elastomeric seal. The jar was thenplaced in an oven for 70 hours at 250° F., removed from the oven,righted, and allowed to cool to room temperature for two hours. Theouter surface of the seal was then examined for evidence of oil weepageand observations were recorded. To quantify weepage, the gross weight ofthe assembly was determined to 0.005 grams. The outside surface of thecircular seal was then wiped clean with a tissue, and the weight wasagain determined. The difference in weight was oil weepage and wasrecorded in grams.

Samples of 45 durometer methyl vinyl silicone rubber cured sheetscompression molded from commercial gasketing compound were encapsulatedwith a 0.5-1.0 mil thick cured coating of fluorosilicone per theteaching of Example 3 and compared to uncoated samples of the samemethyl vinyl silicone rubber using the weepage test method disclosedabove. Thickness of the methyl vinyl silicone molded sheet was 75-80mil, and the test fluid was Ford 5W-30 Factory Fill Motor Oil. Inaddition, this test was also run on 75-80 mil molded sheets of solidfluorosilicone elastomer and on 75-80 mil molded sheets of a 65durometer, high specific gravity, highly filled methyl vinyl siliconeelastomer with the following results:

    ______________________________________                                                       Weepage                                                        Sample Identification                                                                        (grams)   Surface Appearance                                   ______________________________________                                        Methyl vinyl silicone                                                                        <0.005.sup.                                                                             No visible liquid                                    gaskets compound                                                              with 0.5-1.0 mil coating                                                      of fluorosilicone                                                             elastomer per Ex. 3                                                           Uncoated 45 durometer,                                                                       0.057-0.06                                                                              Wet with large beads of oil                          methyl vinyl silicone                                                         gasketing compound                                                            Solid fluorosilicone                                                                         0.001     No visible liquid                                    elastomer                                                                     Uncoated, 65 durometer                                                                       0.020     Wet with small beads of oil                          highly filled, high specific                                                  gravity, methyl vinyl silicone                                                compound                                                                      ______________________________________                                    

The results demonstrate that fluorosilicone coating imparts weepageresistance to methyl vinyl silicone substrates essentially equivalent tothat of solid fluorosilicone elastomer. Subsequent to oil exposure, thefluorosilicone coating continued to adhere tenaciously to the methylvinyl silicone substrate with no evidence of delamination ordeterioration.

In addition, the results indicate that for uncoated methyl vinylsilicone compounds, weepage decreases as a function of filler loadingand specific gravity but is still visible and significantly greater thanthat for fluorosilicone coated methyl vinyl silicone substrates. Whereashighly filled, high specific gravity silicone rubber compounds haverelatively poor physical properties with moderate weepage,fluorosilicone coated methyl vinyl silicone compounds display negligibleweepage without incurring the adverse effects on physical properties ofhigh filler loadings.

EXAMPLE 6

    ______________________________________                                        Effect of Fluorosilicone Coating on Physical Properties                       of Silicone Rubber                                                                           Uncoated                                                                             Coated (Ex. 3)                                          ______________________________________                                        Shore A Durometer (pts.)                                                                        43       45                                                 Tensile Strength (psi)                                                                         1303     1277                                                Elongation (%)   778      821                                                 100% Modulus (psi)                                                                             165      155                                                 ______________________________________                                    

The results clearly indicate that the fluorosilicone coating hasnegligible effect on the fundamental physical properties of the methylvinyl silicone elastomer substrate.

Although specific examples of the invention have been described herein,it is not intended to limit the invention solely thereto but to includeall variations and modifications falling within the spirit and scope ofthe appended claims.

What is claimed:
 1. A method for substantially reducing oil permeability of silicone rubber, comprising the steps of(a) applying a curable fluorosilicone coating to a silicone rubber substrate; and (b) curing the fluorosilicone coating on the substrate by means of a catalytic amount of a platinum group hydrosilation catalyst and a catalytic amount of a peroxide catalyst.
 2. The method as defined in claim 1, wherein the fluorosilicone coating further comprises vinyl-containing fluorosilicone and hydrogen-containing siloxane.
 3. The method as defined in claim 2, wherein the peroxide catalyst is benzoyl peroxide.
 4. The method as defined in claim 1, wherein the fluorosilicone coating has more than 80 mole percent fluorosilicone content.
 5. The method as defined in claim 4, wherein the fluorosilicone coating has more than 95 mole percent fluorosilicone content.
 6. The method as defined in claim 5, wherein the fluorosilicone coating has more than 99 mole percent fluorosilicone content.
 7. The method as defined in claim 1, wherein the fluorosilicone coating further comprises a nonreactive solvent.
 8. The method as defined in claim 7, wherein the nonreactive solvent is selected from the group consisting of halogenated solvents, esters, ketones and ethers, and mixtures thereof.
 9. The method as defined in claim 1, wherein said step (a) further comprises the step of spraying the fluorosilicone coating onto the substrate.
 10. The method as defined in claim 1, wherein said step (b) further comprises the step of curing the fluorosilicone coating by UV light.
 11. The method as defined in claim 1, wherein the cured fluorosilicone coating has a thickness of 0.1-10 mil.
 12. The method as defined in claim 11, wherein the cured fluorosilicone coating has a thickness of 0.5-3.0 mil.
 13. The method as defined in claim 1, wherein there is additionally present a filler. 